1. Field of the Invention
The present invention generally relates to the field of semiconductor substrate processing equipment. More particularly, the present invention relates to a gas distribution system which provides separate and uniform delivery of two or more gases into a processing chamber.
2. Background of Related Art
In the fabrication of integrated circuits, vacuum process chambers are generally employed to process semiconductor substrates. The processes carried out in the vacuum chambers typically provide the deposition or etching of multiple metal, dielectric, and semiconductor layers on the surface of a substrate. Examples of such processes include chemical vapor deposition (CVD), physical vapor deposition (PVD), and etching processes. Many processing chambers include a gas distribution system to effectuate depositions, etching, and so forth. Dry etching of semiconductor materials can also be conducted with chemical vapor transport systems to selectively remove desired areas of such materials to form a desired pattern or configuration on a substrate.
To better understand the integration of a gas distributing system in a processing chamber, FIG. 1 is a schematic diagram showing the construction of a typical CVD chamber 10. The chamber 10 generally defines a processing region 12 and houses a vertically movable substrate support member 14. Containers 20 are provided to supply a variety of gases needed to carry out the processes in the chamber 10. A vaporizer 22 is provided to heat and vaporize one or more liquid precursors, while a flow controller 24 governs the rate at which gases are delivered into the chamber 10. For processes requiring multiple gases, the chamber 10 typically includes an upstream premixing chamber 26, wherein process gases are combined prior to being introduced into the processing region 12.
Gas delivery into the chamber 10 is accomplished by a gas distribution assembly 28, shown in detail in FIG. 2, consisting of a gas manifold 30, a gas box 32 (or gas injection cover plate), a showerhead assembly 34, and an isolator 36, all of which are mounted on an electrically grounded chamber lid 38. The showerhead 34 typically comprises a perforated blocker plate 40 and a faceplate 42 having an array of holes 44. Both the blocker plate 40 and the faceplate 42 are generally flat circular members through which gases are diffused or passed to provide a uniform concentration of gases over the substrate surface. A cavity between the blocker plate 40 and the gas box 32 also serves as an additional agitation stage to continue mixing the process gases. O-rings 46 are disposed between the various components to help ensure hermitic seals to prevent leakage of the gases. In operation, the process gases are pumped into the CVD chamber 10 to effectuate deposition onto a substrate.
Improvements in gas distribution systems are needed, because as integrated circuit density increases and feature size decreases, new materials having low dielectric constants in plasma-less deposition are being developed. In the area of dielectrics, for example, silicon dioxide (SiO.sub.2) is formed by mixing methylsilane (SiH.sub.3 CH.sub.3) and hydrogen peroxide (H.sub.2 O.sub.2). These chemicals undergo a condensation reaction on a cooled substrate to form a porous oxide network.
Critical to the use of H.sub.2 O.sub.2 and SiH.sub.3 CH.sub.3 is keeping the chemicals separate during delivery into the chamber to prevent them from reacting prior to their introduction into a processing region of a vacuum chamber. Allowing a reaction to occur at any point upstream of the processing region results in clogging of equipment components, such as the faceplate 42 and blocker plate 40 of a vacuum chamber gas distribution assembly. Once the gas distribution plates are obstructed, the gases no longer uniformly distribute across the surface of the substrate and nonconformal deposition patterns can result, thereby producing defective devices. In order to clean the gas distribution plates, production must be interrupted and the gas delivery system must be disassembled, serviced, and reassembled.
One attempt to provide a dual channel delivery showerhead is shown in U.S. Pat. No. 5,624,494 entitled "Showerhead for a Gas Supplying Apparatus." The showerhead arrangement disclosed includes two separate perforated plates, each providing a separate gas pathway, joined at an interface. A first set of holes is formed in both a top plate and a bottom plate and must be aligned when the plates are assembled to form a continuous channel through both plates. A second pathway is defined by a third set of vertical holes formed in the bottom plate and fluidly connected by a set of horizontal recesses also formed in the bottom plate. Concentric gas pipes separately deliver reactive gases to the first and second pathways, which then route the gases into a process chamber for mixing and deposition on to a substrate.
One difficulty encountered with this gas plate is achieving the alignment of the holes formed within the two perforated plates. This alignment is critical and is difficult to achieve. Another problem is ensuring a gas-tight seal between the plates to prevent leakage between the holes. As a consequence, the reactive gases migrate through the interstitial spaces formed between the plates and holes and deposit thereon. For processes involving reactive gases, such as H.sub.2 O.sub.2 and SiH.sub.3 CH.sub.3, the resulting chemical compound clogs the gas delivery system and ultimately leads to a non-uniform deposition pattern on the substrates. Furthermore, the blockage requires, at a minimum, cleaning of the showerhead. Substantial blockage may also use upstream pumping equipment and require their maintenance or replacement.
A need, therefore, exists for a gas distribution assembly which provides for separate introduction of gases into a process chamber which uniformly delivers gases onto a substrate without the above blockage problems.